Conventional plasma gun nozzles (anodes) used in thermal spray applications have a limited life. As long as the plasma voltage is maintained in a predefined range for proper operation, the nozzle is operational. However, as exposure to the plasma arc deteriorates the nozzle wall, the plasma voltage drops, as does the life of the nozzle. Typically, the nozzle life is under 40 hours. Moreover, during gun operation the walls are subjected to a number of other conditions that result in voltage decay and arc instability, e.g., cracking of the Tungsten lining used in some nozzle designs.
What is needed is a nozzle and method of producing such a nozzle that minimizes the affects of conditions that result in voltage decay and arc instability.
Generally, there are two key characteristics for controlling the attachment of the plasma arc to the nozzle walls. Charge concentration, as described, e.g., in U.S. Pat. Nos. 7,030,336 and 4,841,114, the disclosures of which are expressly incorporated by reference herein in their entireties, can be used to drive the attachment of the plasma arc to a particular location. However, to control plasma arc attachment in this manner requires a change in gun geometry that could affect the operating conditions for existing plasma guns used to spray a number of existing applications.
The second characteristic for controlling the plasma arc attachment point is the thermal state of the nozzle walls. It has been found that hotter surfaces and boundary conditions are more attractive to the plasma arc, while cooler surfaces and boundary conditions are less attractive to the plasma arc, see, e.g., International Publication No. PCT/US2012/022897, the disclosure of which is expressly incorporated by reference herein in its entirety. Thus, in this manner, it is possible to improve gun performance in terms of voltage stability and controlling voltage decay by applying the thermal management techniques to provide preferred wall conditions for plasma arc attachment.
Heretofore, the design of plasma gun nozzles has been achieved primarily through empirical data, especially with respect to the cooling. These designs have concentrated on providing maximum cooling affects uniformly in the region of plasma arc attachment along the entire plasma nozzle bore.